With an increase in concern for the environmental issues, energy storage devices such as secondary batteries and electric double layer capacitors used for power supply for hybrid vehicles have been actively developed. As the energy storage devices, a lithium-ion battery and a lithium-ion capacitor having high energy performance have attracted attention. The lithium-ion battery, which is compact but can store a large amount of electricity, has been already mounted on a portable information terminal such as a mobile phone or a laptop personal computer, and has helped miniaturization of products.
The secondary battery and the electric double layer capacitor have a structure in which an electrolyte is provided between a positive electrode and a negative electrode. It is known that each of the positive electrode and negative electrode includes a current collector and an active material provided over the current collector. For example, in a lithium-ion battery, a material capable of injection and extraction of lithium ions is used in each electrode as an active material, and an electrolyte is provided between the electrodes.
A lithium oxide and the like are known as a positive electrode active material of a lithium-ion battery (see Patent Document 1).
In the case where the conductivity of a positive electrode active material is insufficient, the conductivity can be increased by coating the positive electrode active material with carbon having a thickness of approximately 5 nm to 30 nm. However, when the positive electrode active material is coated with carbon which does not contribute to charge and discharge, the capacity per volume and weight of a manufactured energy storage device is decreased. The capacity is also decreased in the case where a conduction auxiliary agent is provided, which fills a space in the positive electrode active material and ensures electrical conduction.